This invention relates to a pass filter arrangement comprising (a) a first resonant circuit comprising a first inductive element and a first capacitance, (b) a second resonant circuit comprising a second inductive element and a second capacitance, and (c) a bilateral coupling between said inductive elements.
Bandpass filters formed from discrete inductors and capacitors normally comprise a pair of series or parallel resonant circuits (which may have the same or different resonant frequencies) coupled together by a reactance such as a capacitor, a discrete inductor or the mutual inductance between the inductors included in the two resonant circuits. In general, if the resonant frequencies of both resonant circuits are the same and it is required that the pass band of the filter is to be wider than that which would be given by a combination of the two resonant circuits acting individually, the coupling has to satisfy the requirements that it is inverting over the total path from each inductor back to that inductor via the other inductor (which requirement is met when the coupling comprises a single capacitor or inductance because a signal passing through the capacitor or inductance and then back again experiences two phase shifts of 90.degree. in the same sense in succession).
There is considerable interest at the present time in the replacement of discrete inductors by active circuits which can be constructed in integrated circuit form. For example, it is known that an inductor can be replaced by the driving point impedance of a capacitively-loaded gyrator circuit, i.e. the driving point impedance of a comparatively loaded two-port for which the admittance matrix is ideally ##EQU1## If the inductors in the resonant circuits of the aforementioned bandpass filters are replaced by such active circuits it is still necessary to provide the coupling between the inductors, and if the resonant frequencies of both resonant circuits are the same and it is required that the passband of the filter is to be wider than that which would be given by a combination of the two resonant circuits acting individually, this prima facie requires the provision of a further inductor (which may be simulated) or a further capacitor.
An article entitled "Feedback IF amplifiers for television" by H. S. Jewitt in "Wireless World," December 1954, page 609 et seq. in particular FIG. 2 on page 609, discloses a bandpass filter circuit which comprises a pair of parallel resonant circuits (formed using discrete inductors) with a non-reciprocal bilateral coupling between them, which coupling does not include elements having substantial reactance at the resonant frequency of either of the two resonant circuits. More specifically, in this known filter circuit the coupling comprises an inverting amplifier coupling one resonant circuit to the other resonant circuit, and a resistive coupling from said other resonant circuit back to said one resonant circuit. Thus the coupling is inverting over the total path from each inductor back to that inductor via the other inductor, as required. It will be obvious that making the coupling non-reactive in this or a similar way can itself be attractive when attempting to construct a bandpass filter in integrated circuit form, as such a coupling, at least in principle, does not require any capacitors or inductors.
It is known that a gyrator can be formed by interconnecting a pair of voltage-controlled current sources, one of which is inverting from input to output and the other of which is non-inverting from input to output, in such manner that the input of each source is connected to the output of the other source, each resulting commoned input of one source and output of the other source constituting one port of the gyrator. The coupling between the two resonant circuits in the filter disclosed in the aforesaid article in "Wireless World," as mentioned previously, is formed by an inverting signal path from one inductor to the other inductor, and a non-inverting signal path from said other inductor to said one inductor, and it will be appreciated that a gyrator constructed from a pair of voltage-controlled current sources in the manner outlined above also has this property. If, therefore, the coupling in the filter disclosed in the "Wireless World" article is replaced by a gyrator, and if each inductor therein is replaced by the driving point impedance of a corresponding capacitively-loaded gyrator, a bandpass filter will still be obtained. This filter will be as shown in FIG. 1 of the accompanying diagrammatic drawings, in which one parallel resonant circuit is constituted by a capacitor 1 in parallel with an inductive element comprising the driving point impedance of a gyrator 2 loaded by a capacitor 3, the other parallel resonant circuit is constituted by a capacitor 4 in parallel with an inductive element comprising the driving point impedance of a gyrator 5 loaded by a capacitor 6, and the coupling between the two inductive elements is constituted by a gyrator 7. Such a construction for a bandpass filter is attractive when it is desired to construct the filter in integrated circuit form as, at least in principle, only four capacitors and no inductors are required.
However, it is desirable that a circuit constructed in integrated circuit form should contain as few circuit elements as possible (in order to minimize the chip area required) and that the circuit should be designed so that its overall properties are as little sensitive as possible to production spreads occurring in the manufacture of the circuit. One way to achieve the latter property is to construct any amplifier which may be present in a balanced manner, for example so that it comprises a long-tailed pair of transistors. In general such an amplifier will have two inputs and two outputs and will be non-inverting and inverting, respectively, from one of said inputs to the first and the second of said outputs, respectively, and will be inverting and non-inverting, respectively, from the other of said inputs to said first and second outputs respectively. As mentioned previously a gyrator may be constructed from a pair of voltage-controlled current sources, one of which is inverting and one of which is non-inverting, the output of each being connected to the input of the other. If each gyrator of FIG. 1 is constructed in this manner, and if each voltage-controlled current source is constructed in a balanced manner, the arrangement of FIG. 2 of the accompanying diagrammatic drawings can be obtained, where the gyrator 2 of FIG. 1 has been replaced by a pair of voltage-controlled current sources 8 and 9, the gyrator 5 of FIG. 1 has been replaced by a pair of voltage-controlled current sources 10 and 11, and the gyrator 7 of FIG. 2 has been replaced by a pair of voltage-controlled current sources 12 and 13.
An undesirable feature of the circuit of FIG. 2 is that each capacitor 1 and 5 has inputs of two of the sources 8-13 connected to it and also outputs of two of the sources 8-13 connected to it, which feature tends to make the overall performance of the circuit unpredictable and somewhat susceptible to production spreads when it is manufactured in integrated circuit form, particularly in view of the fact that each capacitor 3 and 6 has only one such input and only one such output connected to it. In order to overcome this problem, it would be desirable if the couplings between the capacitors 1 and 5, formed by the sources 12 and 13, could be replaced by signal paths from capacitor 1 to capacitor 5 via sources 8 and 11, and from capacitor 5 to capacitor 1 via sources 10 and 9, respectively, in particular by coupling the "unused" output of source 8 to the "unused", i.e. grounded input of source 11 and by coupling the "unused" output of source 10 to the "unused" i.e. grounded input of source 9. Such a measure would also result in considerable simplification of the arrangement. However, it will be seen that taking these steps will result in a non-inverting signal path from capacitor 1 to capacitor 5 and in a non-inverting signal path from capacitor 5 to capacitor 1, rather than in signal paths the other of which is non-inverting and one of which is inverting as required. It is of course possible to modify the circuit of FIG. 2 by making the source 8 inverting and the source 9 non-inverting and/or by making the source 10 non-inverting and the source 11 inverting but consideration of the various possibilities given by these modifications reveals that attempting to replace the sources 12 and 13 by pairs of the sources 8-11 together with couplings between "unused" inputs and outputs thereof will still always result in couplings between the capacitors 1 and 3 which are either inverting in both directions or non-inverting in both directions.